The silicon photovoltaic industry widely uses screen printing of aluminum/silver (Al/Ag) based paste for grid metallization. The main disadvantage of this technology is the difficulty in making smaller conduction lines and the high contact resistance of the paste conductor. The smallest width of screening printed conduction lines is on the order of 100 microns. In addition, screen printing can only print low aspect ratio features, which translates into wider conduction lines in order to provide sufficient conductivity, which means more shadowing for the front-side grid. Al/Ag paste is also a very expensive material and the screen printing process is the leading cause of wafer cracking in the solar cell building processes.
A method described in U.S. Pat. No. 7,339,110 to Sunpower uses vacuum based processes to deposit a seed layer and the barrier layer before Cu plating. The combination of vacuum and solution processes for the conventional solar cell is cost prohibitive. Thus, Sunpower uses it in the interdigitated single crystalline silicon solar cells, which will yield higher efficiency at a higher cost to justify the high cost processes.
Solar cells with interdigitated contacts of opposite polarity on the back surface of the cell generally differ from conventional solar cells with front side metal grids and blanket or grid metallized back side contacts. These solar cells improved photo-generation due to elimination of front grid shading, reduced grid series resistance, and improved blue photo-response since heavy front surface doping is not required to minimize front contact resistance because there are no front contacts. As described in U.S. Pat. No. 4,927,770 to Swanson, the back-contact cell structure allows simplified module assembly due to coplanar contacts.
While interdigitated back-contact (IBC) solar cells have been fabricated, cost considerations have prevented the IBC solar cell from commercialization. Thus, conventional microelectronics (integrated circuit) processing has been employed in fabricating IBC solar cells, including the use of backside diffusions, contacts, and metal lines fabricated by conventional microelectronics photolithography, thin film metallization, and etching processes. However, the process is not cost effective for application in conventional low-cost, flat-plate solar panels.